1. Field of the Invention
The present invention relates to integrated circuit technology in general and more particularly to integrated circuits for processing analog signals within the millivolt range.
2. Prior Art
The use of receiver circuits for extracting information in the form of electrical signals from a communications network is well documented in the prior art. Such receiver circuits are available as discrete components on a card or integrated components on a single chip. The integrated version of those circuits are preferred over the non-integrated version in that, among other reasons, the single chip occupies much less space than a card.
Even though the single chip receiver is attractive in many ways, it creates several technical problems which demand special and unique solutions. Oftentimes, the problems are created as a result of the environment in which the chip is used. If the chip is used to process digital signals, the problem is minuscule since a purely digital process, such as CMOS, can be used to fabricate it. However, when the chip is used to process mixed signals (analog and digital) the problem is how to use digital components and a digital processing technique, such as CMOS, to provide both analog and digital functions on a single chip.
There are several environments in which the signals to be processed are within the millivolt range. As will be explained subsequently, such small signals raise additional problems which must be cared for if the integrated chip is designed for use in these environments. Thus, the concern is not only providing the analog and digital function on a single chip but also processing signals within the millivolt range. Among the environments in which such small analog signals are prevalent is the Local Area Network espoused by IEEE 802.5 Committee (IEEE Standard Token Ring ANSI/IEEE Standard 8802/2 and 5 - 1985). Details of the network are set forth therein and are incorporated herein by reference.
Designing the single chip so that the receiver circuit can be used in noisy environments present another problem for the designer. It is well known that hysteresis, provided by a Schmitt trigger, can be used to make a receiver circuit immune to noise. Essentially, the hysteresis phenomenon, be it introduced by a Schmitt trigger or other types of electrical circuits, sets the switching point for an output signal. Even though the hysteresis phenomenon is well known, the problem is how to implement it on a single chip with mixed functions (analog and digital). The prior art has introduced hysteresis characteristics with resistors, lamda diodes and extraneous process steps. All of the prior art techniques increase the overall cost of the chip.